It is known that a glass material that has been softened by heating can be press molded with a pressing mold to transfer the shape of the molding surface to the glass material, thereby producing an optical element such as a glass lens. The optically functional surfaces of an optical element that has been formed by press molding possess the desired optical performance without being subjected to mechanical processing such as polishing.
Patent Reference 1 discloses a method of manufacturing pressed lenses by placing a glass material (preform), the surface of which has been coated with a silicon oxide film, in a pressing mold, and press molding the glass material at a temperature at which it is in a softened state. This method is described as preventing fusion of the mold and glass during press molding, and preventing the precipitation of reduced particles due to reduction of PbO contained in the glass. According to the invention described in Patent Reference 1, the practical range of the thickness of the silicon oxide film is 50 to 2,000 Angstroms (5 to 200 nm). At below 50 Angstroms (5 nm), the effect of silicon oxide film formation is not achieved, and at greater than 2,000 Angstroms, defects such as cracks tend to occur during press molding, compromising optical qualities such as transmittance and the refractive index.
Patent Reference 2 discloses a method for molding optical glass elements by forming a thin layer comprised primarily of SiO2 in the center portion of the surface of a glass element in advance, and placing the glass material in a pressing mold and conducting press molding. This method is considered to yield an optical element having an optically functional surface such that the occurrence of surface cracks is avoided and no clouding is generated in the center portion. According to the invention described in Patent Reference 2, the thickness of the thin film comprised principally of SiO2 is 100 to 200 Angstroms. At below 100 Angstroms (10 nm), there is no cloud-preventing effect, and outer layer cracking occurs at 200 Angstroms and above.
Patent Reference 3 discloses an optical element in which a double outer surface layer is formed on the outer surface of a preformed glass core, with a first outer surface layer closer to the glass core being formed as a film on the outer surface of the glass core at a temperature at or above the glass transition temperature with the glass core material in a state of reduced pressure, and a second outer surface layer farther from the glass core being formed as a film on the first outer surface layer by sputtering or the like a glass material for vapor deposition. Patent Reference 3 also discloses a method for manufacturing the optical element. The glass material for vapor deposition that is used to form the second outer surface layer is a glass material comprising SiO2, Na2O, Al2O3, and B2O3. According to the invention, even when highly reactive glasses and glasses having readily volatizing glass components are employed as the glass material, it is possible to obtain an optical element without the occurrence of surface cracking that is free of fusion and clouding. According to the invention described in Patent Reference 3, the second outer surface layer comprised principally of SiO2 is 5 to 50 nm in thickness. At less than 5 nm, the fusion-preventing effect (between mold and glass) was found to decrease, and at 50 nm and above, cracking occurred.